Jenson Button admitted he was struggling to get an adequate rear-end balance on his McLaren after ending Friday practice in Spain in fourth place.
The Briton was 0.7 seconds off the lead pace and conceded that with his car's current handling he could not match his second-placed team-mate Lewis Hamilton.
"I'm not particularly happy with the balance of the car at the moment," said Button.
"I'm really struggling with the rear end - it's pretty unpredictable. It's not something that I like, and it's something that I've got to work on and improve for tomorrow.
"If I look at what Lewis is doing, I just don't have the balance and I'm a little way off at the moment. There's a bit more pace in the car, we've just got to unleash it and get a balance that I like."
He was also sceptical about whether McLaren had closed in on Red Bull at all this weekend.
"I wouldn't say we've seen anything yet to prove that. We all hope, but I can't see anything yet," said Button.
"They're never that quick on a Friday. I was quickest on Friday at the last race, so it's actually that they've got quicker.
"Tomorrow they'll show their true pace and I'm guessing they're going to have half a second on everyone."
Despite getting within 0.039s of pacesetter Mark Webber's Red Bull, Button's team-mate Hamilton agreed that the champion team probably still had an edge.
"I think we are just as competitive as we have been in the past, but that doesn't mean that we are actually close to them," he said.
"We'll wait and see tomorrow. It depends on what their engine mode is on and all those kind of things. We don't know. We might have a little bit in the bag, they might have a little bit in the bag. We wait and see."